The separators are slightly off from the proton specs.
Light theme should be: #F0F0F4
Dark theme should be: #52525E.
They are currently using these values: https://searchfox.org/mozilla-central/search?q=--panel-separator-color%3A&path=&case=false®exp=false. The current value can be used as a fallback.
Note this change makes the separators harder to see.
I suggest a lower priority (P3?) for this work